Sheet material for radio wave-absorbing body and radio wave-absorbing body

ABSTRACT

A sheet material for a radio wave absorber and a radio wave absorber formed from the sheet material, where the sheet material is light weighted and has excellent form-retaining capability and workability for field assembling. A sheet material ( 1 ) for a radio wave absorber has a corrugated paperboard structure with an undulated corrugated medium ( 2 ) and a planar liner ( 3 ) that are layered over each other. The corrugated medium-( 2 ) and/or the liner ( 3 ) are constructed from a sheet including an electrical-loss material. A radio waveabsorber ( 10 ) is characterized in that the sheet material ( 1 ) for a radio wave absorber is cut, folded, and assembled as a hollow three-dimensional structure body, which has a shape of wedge, polygonal pyramid, or polygonal cylinder.

TECHNICAL FIELD

The present invention concerns a sheet material for a radio waveabsorber and a radio wave absorber made of this sheet material forassembly and, more particularly, a sheet material for a radio waveabsorber that can be used in the walls, ceiling and floor of an anechoicchamber and a radio wave absorber thereof.

BACKGROUND ART

The anechoic chamber is used for measurement testing the properties ofantennae or radio-wave measurement testing an electronic device.

A radio wave absorber is fitted to the walls, ceiling, floor and so onof the anechoic chamber used in such measurements so as to shield theinside from external radio wave and, at the same time, prevent any radiowaves generated from the internal device to be measured from radiatingexternally.

Many of radio wave absorbers used for such a purpose have been made ofresin foam such as urethane foam, styrene foam and so on that isimpregnated with carbon black, an electroconductive material and thenshaped into the wedge shape or pyramidal shape.

However, the radio wave absorber molded with the resin foam in this wayis bulky and, moreover, fragile with the corner ends being broken byvibrations during the transportation or if they are knocked againstsomething. And this increases the necessary storage space and hence thecost of storage, and a large volumetric capacity is required for thepacking in order to prevent it from being broken during transportation,increasing transportation cost and thus increasing the overall cost.

To counter act such problems, it has been proposed taking the carbonblack containing plate material to the site of execution and thenassembling it into a radio wave absorber of hollow pyramidal shape orsuch like (Japanese patent application Kokai publication No. 1999-87978,Japanese patent application Kokai publication No. 2000-216584 and soon).

However, this carbon black containing plate material should be about 5to 20 mm thick, because if it is too thin, distortion or shapeinstability can occur in the radio wave absorber after assembly, due tolack of rigidity. Howsoever, if the plate material is made thick, itsweight also increases, deteriorating onsite workability, making itdifficult to reduce transportation cost, increasing the need for carbonblack, and various other such problems.

On the other hand, as an example of avoiding the weight increase of theaforementioned plate material, it has been proposed to form theaforementioned plate material into a honeycomb structure body with anumber of cells arranged in a simple pattern (Japanese patentapplication Kokai publication No. 2000-77883).

However, the radio wave absorber assembled from this plate material ofthe honeycomb structure body has a problem of deteriorating theintrinsic functions as a radio wave absorber, because transmission ofhigh-frequency electromagnetic wave is facilitated due to a structurewhere the cell opening is served as the radio wave incident face whenthe cell apertures of the honeycomb structure are large. Consequently,if this problem is to be avoided, it is necessary to reduce the cellsize. As a result, the weight is increased and therefore such problemsof increasing the weight in the aforementioned Japanese patentapplication Kokai publication No. 1999-87978 and Japanese patentapplication Kokai publication No. 2000-216584 can not be essentiallyresolved.

And though the plate material of the aforementioned honeycomb structurebody has rigidity against load in the thickness direction, themechanical strength against load in a direction orthogonal to thethickness direction may lack in some cases, and hence it can notnecessarily be said to improve the assembly workability on site.

On the other hand, it has been proposed to assemble a radio waveabsorber, through heat fusing the ends of a plastic paperboard moldedfrom thermoplastic resin with carbon black added (Japanese PatentPublication No. 2760578).

However, the manufacturing cost becoming extremely high can not beavoided, because plastic paperboards are manufactured through extrusionmolding. In addition, there is a problem with on site assemblyworkability, because assembly requires local heating and softeningtreatment. Moreover, when carbon black is used as the radio waveabsorbing material, radio wave loss tends to decrease as the frequencybecomes higher. Therefore, there is the drawback that a radio waveabsorber designed for an EMC anechoic chamber utilizing 30 MHz to 1 GHzas the main frequency range does not provide a sufficient absorbingproperty in microwave or milliwave ranges equal or higher than the same.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a sheet material for aradio wave absorber that can solve the aforementioned problems ofconventional technologies while presenting excellent radio waveabsorbing capacity and, moreover, being still light weighted withexcellent form-retaining property and workability for field assembly,and to provide a novel radio wave absorber formed from the same.

The sheet material for a radio wave absorber of the present inventionfor use in attaining such an object has a following composition.

That is, the sheet material for a radio wave absorber of the presentinvention is characterized by having a paperboard structure with acorrugated medium and a planar liner that are layered over each other,wherein the corrugated medium and/or the liner are composed of sheetsthat includes electrical-loss material.

In addition, the radio wave absorber of the present invention for use inattaining the aforementioned object has a following composition.

That is, the radio wave absorber of the present invention ischaracterized in that the aforementioned sheet material for a radio waveabsorber is cut, folded, and assembled into a hollow three-dimensionalstructure body, which has a shape of wedge, polygonal pyramid, orpolygonal cylinder.

The sheet material for a radio wave absorber used in the presentinvention as mentioned above is light weighted with a hollow section andcan be handled in a sheet state, thus facilitating storage,transportation and workability of field assembling, because it is basedon the paperboard structure with the corrugated medium and a planarliner layered over each other.

In addition, it can improve the form-retaining property of the radiowave absorber after assembly, because, although light weight, it hasappropriate rigidity from the corrugated medium.

As mentioned above, the sheet material for a radio wave absorber of thepresent invention, being based on the paperboard structure, is lightweighted, presents a moderate rigidity, allows to reduce thetransportation cost because it can be transported as a sheet andassembled easily in the field and, at the same time, facilitates theassembly work into the radio wave absorber in the field.

In addition, the storage cost can be reduced, because it can be storedcompactly as a sheet.

Furthermore, the radio wave absorber of the present invention does notcause distortion not only during the assembly but also after theassembly, because the sheet material presents a moderate rigidity,allowing to maintain an excellent shape stability for a longtime.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view showing an example of sheetmaterial for a radio wave absorber of the present invention;

FIG. 2 (A) to (D) are respectively sectional views showing examples of asheet material for a radio wave absorber of the present invention;

FIG. 3 is a plan view showing an example of cutting of sheet materialfor a radio wave absorber of the present invention;

FIG. 4 is a perspective views of a radio wave absorber assembled fromthe sheet material for assembly of FIG. 3;

FIG. 5 (A), (B) are plan views showing examples of cutting of sheetmaterial for a radio wave absorber of the present invention;

FIG. 6 is a perspective view of a radio wave absorber assembled from thesheet material for assembly of FIG. 5 (A), (B);

FIG. 7 is a perspective view of a radio wave absorber according toanother embodiment of the present invention;

FIG. 8 is a perspective view of a radio wave absorber according to stillanother embodiment of the present invention;

FIG. 9 is a perspective view of a radio wave absorber according to stillanother embodiment of the present invention;

FIG. 10 shows a radio wave absorber of a pyramidal shape illustrating anembodiment of the present invention;

FIG. 11 shows a radio wave absorber of a wedge shape illustrating anembodiment of the present invention;

FIG. 12 is a cross sectional view of the radio wave absorber shown inFIGS. 10 and 11, as their shapes of cross-section are identical, onefigure is used to explain them;

FIG. 13 is a cross sectional view showing an embodiment in the case thattwo inner radio wave absorbers are arranged in FIGS. 10 and 11, as theirshapes of cross-section are identical, one figure is used to explainthem;

FIG. 14 shows a radio wave absorber of a pyramidal shape according to anembodiment of the present invention;

FIG. 15 shows a block diagram of an inner radio wave absorber used forthe radio wave absorber of the pyramidal shape shown in FIG. 14;

FIG. 16 shows a radio wave absorber of a wedge shape in an embodiment ofthe present invention;

FIG. 17 shows an embodiment in the case that two inner radio waveabsorbers are arranged in the embodiment shown in FIG. 16;

FIG. 18 shows another embodiment of the radio wave absorber of a wedgeshape according to an embodiment of the present invention;

FIG. 19 shows one unit of a radio wave absorber composed by arrangingfour radio wave absorbers of a wedge shape shown in FIG. 18 so that theridge lines of their wedge alternately cross; and

FIG. 20 shows an example of a radio wave absorbing property of the oneunit shown in FIG. 19.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a sheet material for a radio wave absorber of the presentinvention and a radio wave absorber of the same will be furtherdescribed with reference to the drawings etc.

FIG. 1 illustrates a sheet material for a radio wave absorber of theinvention.

The sheet material 1 for a radio wave absorber of the present inventionhas a paperboard structure with a corrugated (bend into wave-like form)medium 2 sandwiched therebetween and a planar liner 3, 3 that arelayered on the both sides thereof. Crest portions and valley portions ofthe corrugated medium 2 bent into wave-like form are adheredrespectively to the liner 3, 3 through an adhesive. Moreover, a sheetincluding an electrical-loss material is used for the corrugated medium2 and/or liner 3 and preferably a mixed paper including carbon fiber isused. Thus, the sheet material 1 used for assembly presents an excellentradio wave-absorbing property by mixing the electrical-loss material.

The sheet material for a radio wave absorber of the present inventioncomposed as mentioned above is cut into a predetermined shape and,thereafter, assembled into a radio wave absorber of hollowthree-dimensional structure (described in detail below) on the executionsite. With the hollow structure, this sheet material for a radio waveabsorber is light weighted and, at the same time, presents a moderaterigidity by involving the corrugated medium and keeps an excellentform-retaining property even after being assembled as a radio waveabsorber. In addition, it is not bulky, because it can be stored ortransported in a planer sheet state, allowing to reduce the transportcost.

In the sheet material for a radio wave absorber of the presentinvention, the sheet including the electrical-loss material is usedadvantageously for both of corrugated medium and liner of the paperboardstructure, but it may also be composed to be used for one of them. Inthe case of using it for one of them, it may be preferably used for thecorrugated medium.

The structure of the paperboard is not specially limited; however, it ispreferably selected from single faced paperboard, double facedpaperboard, double wall paperboard and triple wall, to obtain a sheetthat is thin, light weighted and strong as much as possible.

Here, the single faced paperboard designates a paperboard structurewhere a corrugated (undulated) medium 2 is affixed to a single liner 3,as shown in FIG. 2(A), the double faced paperboard designates apaperboard structure where a corrugated (undulated) medium 2 is joinedbetween two liners 3, 3 as shown in FIG. 2(B) and the double wallpaperboard designates a paperboard structure where a single facedpaperboard is jointed to one side of the double faced paperboard, asshown in FIG. 2(C). In addition, the triple wall designates a threelayered paperboard structure where another single faced paperboard isjointed to the double wall paperboard, as shown in FIG. 2(D). Amongthem, the double faced paperboard is preferable, because it is thin andpresents a moderate rigidity at the same time.

As manufacturing method of these paperboards, a rapid well-knownmanufacturing method of paper paperboards offering a low manufacturingcost may be used.

To be more specific, the single faced paperboard can be made bycorrugating the medium by a machine called corrugator, and pasting tothe surface or back of the liner. Further, a mass production method ofpaperboard sheets where a single faced paperboard and liners are adheredand heated to obtain a double faced or a double wall paperboard, that issent into a cutter always in a stable state and cut into a predetermineddimensions as a paperboard sheet can be used.

As an adhesive of the aforementioned paperboard, any of well-knownadhesives such as starch glue can be used.

For the paperboard structure applied to the present invention, it ispreferable to set the thickness t per a layer of the paperboard to 1 to5 mm. If the thickness t is less than 1 mm, the weight will increase,and if it exceeds 5 mm, the volume will increase, deteriorating theworkability to assemble as a radio wave absorber and thetransportability.

Furthermore, in the paperboard structure, it is preferable that the takeup ratio of the corrugated medium to the liner is in a range of 1.2 to 2times, and the interval w between tops of adjacent corrugated mediums isin a range of 1 to 15 mm.

Here, the take up ratio means the ratio of the glued corrugated mediumlength to the liner length, and this take up ratio is preferably in theaforementioned range, considering both the gluing strength and thegluing workability. Moreover, the interval w between tops of adjacentcorrugated mediums is preferably in the aforementioned range,considering both the man-hour required for the gluing step and thestrength.

Different concentration of electrical-loss material can be distributedfor each layer, in the case of structures where a plurality ofpaperboards are laminated, such as double wall paperboard or triplewall. For instance, a concentration gradient can be set by increasingthe concentration of electrical-loss material of the paperboard from theradio wave incident side to the depth side. Radio wave can be absorbedmore efficiently, because radio wave reflection on the surface issuppressed as much as possible and the absorption takes place in thedepth side of the inside, by increasing the concentration ofelectrical-loss material from the radio wave incident side to the depthside gradually in this manner. Moreover, paperboards can be laminatedfrom the radio wave incident side gradually to the depth side byreducing the thickness t thereof. A similar radio wave effect can beobtained by a concentration gradient similar to the aforementioned onefor reducing gradually the thickness t of the paperboard of respectivelayers in this manner.

The electrical-loss material contained in the sheet material of thepresent invention performs the attenuation function by converting radiowave energy into an extremely small current and, furthermore, convertinginto heat energy.

Such electrical-loss material includes, for instance, electroconductivepowders such as carbon black, carbon micro coil powder, graphite powderand so on, electroconductive fibers such as carbon fiber, siliconcarbide fiber, metal fiber, metal plated fiber and so on. It may be asemiconductor fiber obtained by controlling the sintering temperaturewhile producing carbon fiber or silicon carbide fiber.

Electroconductive fibers are particularly preferable among theseelectrical-loss materials and, furthermore, the carbon fiber is morepreferable. The electroconductive fiber, presenting a high aspect ratio(ratio of length to size), permits to obtain a large radiowave-absorbing effect compared to the powder such as carbon black,because fibers come easily in contact with each other even if thequantity is small.

Moreover, for the electroconductive fiber, a loss by the induction ofcurrent also in the electroconductive fiber existing alone is added tothe radio wave loss caused by the contact of adjacent electroconductivefibers each other in a way to let current flow through the whole medium.This phenomenon is principally a resonance phenomenon and, the currentinduced by the electroconductive fiber increases as the fiber length isthe integer number of times of the half wavelength of the wavelength inthe medium. Namely, a phenomenon where the radio wave loss decreases asthe frequency becomes higher as in the carbon black loss material doesnot occur, because the loss by resonance phenomenon is added to theradio wave loss when the electroconductive fiber is used as the radiowave loss material. Therefore, the electroconductive fiber is excellentas loss material for a radio wave absorber that can cover a wide bandrange from low frequency to microwave, milliwave.

Conventionally, the upper limit frequency of the anechoic chamber forEMC was 1 GHz; however, in these years, there is a trend to extend tothe proximity of 10 GHz. In the case of the sheet material for a radiowave absorber using the electroconductive fiber as electrical-lossmaterial in the present invention, a good absorption property can beexhibited in a frequency range of 30 MHz to 1 GHz and also with amicrowave frequency exceeding the same. Furthermore, a sufficient goodabsorption property can be exhibited in a milliwave frequency range upto the proximity of approximately 100 GHz.

The shape of the sheet including the electrical-loss material is notespecially limited provided that the electrical-loss material isdistributed all over the sheet. Preferably, mixed paper made by mixingelectroconductive fiber and non-electroconductive fiber is advantageous,as it can be produced easily.

As non-electroconductive fiber to be mixed with electroconductive fiber,polyester fiber, nylon fiber, glass fiber, aramid fiber,polyphenylene-sulfide fiber, poly(etheretherketone) fiber,polyparaphenylene-benzobisoxasol fiber, polylactic fiber or the like canbe used.

It is preferable to use a fiber presenting especially a volumeresistivity larger than that of its counterpart electroconductive fiberto be mixed together by double figures or more, as this nonelectroconductive fiber.

As for the manufacturing method of mixed paper, any of the wetpaper-making method for paper-making a slurry mixing at least one kindof electroconductive fiber and non electroconductive fiber respectivelyand water and the dry paper-making methods for stirring and mixing inthe air at least one kind of electroconductive fiber and nonelectroconductive fiber respectively and collecting the same in thesheet form may be adopted. In any of the case of wet paper-making methodand dry paper-making method, the mixed paper can be producedcontinuously by using a continuous transfer net conveyer as paper-makingmeans. In these paper-making methods, where necessary, inorganic binderssuch as aluminum hydroxide or organic binders such as starch, polyvinylalcohol, polyethylene, paraffin, acrylic fiber or the like may be added.

It is preferable to use carbon fiber as electroconductive fiber to beused in the case of wet paper-making method, because the low densityfacilitates the paper-making, the consumption is small as the aspectratio can be increased or for other reasons. Here, it is advantageous touse a carbon fiber of which average fiber length is in a range of 1 to60 mm. If the average fiber length is less than 1 mm, fibers hardlyoverlap each other, thus the number of contact points decreases and itbecomes necessary to increase the consumption in order to compensate thedecrease of contact points and, therefore, the manufacturing cost rises.On the other hand, if the average fiber length exceeds 60 mm, fibersbreak easily, and the consumption can not be reduced necessarily.

As the mixed paper obtained as mentioned above, one presenting anappropriate electric conductivity can be obtained easily especially bythe continuous paper-making method.

Concerning the electric conductivity of the mixed paper, it ispreferable that the ratio (y/p) of the maximum electric conductivity (p)thereof and the electric conductivity (y) measured in a directionorthogonal to the measurement direction that has presented the maximumvalue is in a range of 0.35 to 0.95, when the maximum value of theelectric conductivity that the mixed paper has is p. For the mixed papermanufactured by the continuous paper-making method, the electricconductivity measured in the longitudinal direction of the mixed papercorresponds to the maximum value (p), as electroconductive fivers tendto orient in the net conveyer movement direction thereof; therefore,those where the ratio (y/p) with the electric conductivity (y) in thebreadth direction which is orthogonal to the same is 0.35 to 0.95 aredesirable.

When electroconductive fibers are mixed as mentioned above, fibers runeasily parallel to the net conveyer transfer direction and the electricconductivity in the longitudinal direction of the mixed paper tends tobe high. The fact that the electric conductivity presents thedirectional property means that the directional property is generated inthe current fluidity and a phenomenon where the absorbing abilitydiffers according to the direction of the electric field vibration faceof the incident radio wave, namely polarization dependence is provokedin the absorbing ability. However, in practice, absence of polarizationdependence in the absorbing ability being required, it is desirable tolimit the directional property of the electric conductivity as small aspossible and therefore, it is preferable to set the aforementioned ratioof electric conductivity (y/p) in the range of 0.35 to 0.95.

f the ratio of electric conductivity is smaller than 0.35, thedirectional property becomes excessively large, which is practicallyundesirable. On the other hand, if it is lager than 0.95, the fiberorientation becomes excessively arbitrary, the longitudinal strength ofthe mixed paper lowers largely, often provoking paper cut during thepaperboard manufacturing. As for the method setting the ratio ofelectric conductivity to the aforementioned range, it can be achievedeasily by controlling the net conveyer movement speed mentioned above.

The mixture quantity of electroconductive fiber, especially carbonfiber, in the mixed paper is preferably in a range of 0.08 to 20 wt %,and more preferably in a range of 0.2 to 2 wt %. Lower than 0.08 wt %the electrical loss lowers and, consequently, the radio wave absorbingability lowers. On the other hand, over 20 wt %, though the electricloss increases, more radio wave will be reflected undesirably.

Moreover, in the case where the electroconductive fiber is carbon fiber,the content of sizing agent in the carbon fiber outer skin section ispreferably not more than 0.9 wt % to the whole carbon fiber quantityand, most preferably, 0%. In some cases, some sizing agent is providedin the outer skin section in the manufacturing step of carbon fiber. Ifthe content of this sizing agent is excessive, the electric conductivityby superposed fibers each other will be inhibited and, consequently, theradio wave absorbing ability lowers; however, if the consumption ofcarbon fiber is increased for compensation, the manufacturing cost ofthe sheet material increases. Therefore, the content of sizing agent ispreferable to be set as mentioned above.

The sheet such as mixed paper including electric loss material obtainedas mentioned above is made as a sheet material for a radio waveabsorber, by using it for a corrugated medium and/or liner of apaperboard structure to form a paperboard structure body. The sheet suchas mixed paper including electric loss material may be used at least forone of corrugated medium or liner of the paperboard structure and,preferably used for the corrugated medium bent into wave-like form, moreparticularly.

The sheet face may be the radio wave incident face by adopting a sheetmaterial of paperboard structure and an excellent radio wave absorbingability can be exhibited, because radio wave can not be transmitted, asthe case of publicly known honeycomb structures where the cell openingcorresponds to the radio wave incident face. Also, when the sheetincluding electric loss material is used for the corrugated medium,radio waves reflect irregularly and finely by hitting the corrugatedwaveform face in a way to set off each other, allowing to absorb radiowave efficiently.

Besides, in this sheet material for a radio wave absorber comprising thepaperboard structure of the present invention, it is preferable that atleast one indication mean selected from printing of color, pattern orletter, embossing of pattern or letter and so on is executed on at leastone side of the liner surface. Such execution of the indication byprinting or embossing of various colors, patterns and so on permits toprovide rich expressions to the radio wave absorber. As a result, a darkand oppressing atmosphere of the conventional anechoic chamber where ablack or dark blue monochromatic radio wave absorber is affixed can beresolved, contributing to improve the work environment of a measurer.

Furthermore, in the case of onsite assembly of the sheet material for aradio wave absorber as a radio waveabsorber, the assembly workabilitycan be enhanced furthermore, if face and back indication of the sheetmaterial, assembly instructions or the like are printed or embossed onthe surface thereof.

As mentioned above, the sheet material for a radio wave absorber of thepresent invention can exhibit its ability as a radio wave absorber onlyby cutting into a predetermined size and pasting to the execution sitein the sheet state, because it has a paperboard structure where acorrugated medium and a planar liner are layered and, at the same time,the corrugated medium and/or the liner are composed of a sheetcomprising an electrical-loss material. It becomes especially effective,when the corrugated medium is composed of a sheet comprising anelectrical-loss material.

In a still more preferable use embodiment, it is preferable to cut so asto ready to assemble as a hollow three-dimensional structure body and,thereafter, assemble as a radio wave absorber of hollowthree-dimensional structure body on the execution site. Though it ispreferable to cut the sheet material for assembly before transporting tothe execution site, it may also be cut on the site.

The form is not especially limited in the case of assembly as a hollowthree-dimensional structure body and, for instance, a shape of wedge,polygonal pyramid such as quadrangular pyramid, triangular pyramid andso on (pyramidal shape), or polygonal cylinder such as triangle pole,square pole and so on can be adopted. The radio wave-absorbing abilitycan be improved by forming especially the tip as pointedthree-dimensional shape so that the radio wave reflection is reduced.Moreover, if a lattice composed of the sheet material for a radio waveabsorber, or a support is installed inside the hollow three-dimensionalstructure body, this works as the inner radio wave absorber, allowing todramatically enhance the absorbing property of high frequency inparticular.

For example, an inner radio wave absorber can be also constructed byarranging a conductive thin member in parallel to a bottom surface ofthe radio wave absorber inside the hollow three-dimensional structurebody, or an inner radio wave absorber can be constructed by arranging aconductive thin member perpendicularly to the bottom surface of theradio wave absorber inside the radio wave absorber. The inner radio waveabsorber has an advantage that it can absorb electromagnetic wave whichenters inside the structure of pyramid or wedge shape without beingabsorbed in the hollow three-dimensional structure body obtained byfolding work or the like of sheets for a radio wave absorber of anelectrical-loss material. This inner radio wave-absorbing body allowsthe radio wave-absorbing body to increase a radio wave absorptionamount.

Furthermore, inserting the inner radio wave absorber reinforces themechanical strength of the hollow three-dimensional structure body of apyramidal shape and a wedge shape in addition to an advantage ofimproving the radio wave-absorbing property.

Now, concrete embodiments shall be described for the case of assembly asa hollow three-dimensional structure body.

FIG. 3 illustrates a shape after cutting of the sheet material for aradio wave absorber of the present invention and FIG. 4 shows a statewhere this cut sheet material is assembled as a wedge form radio waveabsorber of hollow three-dimensional structure body.

A plurality of folds 4 are embossed on the cut sheet material 1 with apredetermined interval and parallel to wave rows of the corrugatedmedium 2. An insert flap 5, an insert slit 6 and an overlap width 7 areformed on both ends. The folds 4 are formed in parallel to the wave rowsof the corrugated medium 2 as illustrated, and such formation of thefolds 4 facilitates to fold the sheet for assembly 1 along the folds 4without distorting the same.

The aforementioned sheet material for a radio wave absorber 1 can beassembled easily as a wedge form radio wave absorber 10 of hollowthree-dimensional structure body as shown in FIG. 4, by bending alongthe fold 4, introducing the insert flap 5 of one end into the insertslit 6 of the other end and binding the overlap width 7 with adhesive.

FIG. 5 (A), (B) illustrates a shape after cutting of another embodimentof the sheet material for a radio wave absorber of the present inventionand FIG. 6 shows a state where this cut sheet material is assembled as awedge form radio wave absorber of hollow three-dimensional structurebody.

A sheet material for assembly 1A (refer to FIG. 5(A)) for forming awedge outer shape portion of the hollow three-dimensional structure bodyand a sheet material for assembly 1B (refer to FIG. 5(B)) for forming asupport part for supporting a leg of the wedge outer portion.

The sheet material for assembly 1A of FIG. 5(A) is arranged, as shown bythe cutaway view, so that the corrugated medium 2 sandwiched by upperand lower double side liners 3 makes wave rows of the waveform in theright-left direction of the drawing. A fold 4 is embossed approximatelyat the middle crossing the wave rows of the corrugated medium 2 at rightangles.

Besides, folds 4 respectively parallel to the wave rows of thecorrugated medium 2 are embossed on the upper and lower both ends, andan outer shape reinforcing part 11 and an insert slit 6 are formed onthe both outsides thereof.

Also, a fold 4 is embossed at the right end in the drawing so as tocross the wave rows of the corrugated medium 2 at right angles and anoverlap width 7 to the anechoic chamber wall face is formed on theoutside thereof.

The sheet material for assembly 1B of FIG. 5(B) forms a rectangularshape C in the middle and trapezoidal shape D on both sides thereof byembossing folds 4 parallel to the wave rows of the corrugated medium 2respectively at 2 points inside. An outer shape reinforcing part 11 andan insert flap 5 are formed outside both the right and left sides of thetrapezoidal shape D in the 2 places, by embossing folds 4 crossingrespectively the wave rows of the corrugated medium 2 aslant. Only oneside of the insert flap 5 falling on the fold 4 is linked, whileremaining 3 sides are formed to be detached from the outer shapereinforcing part 11. A fold 4 parallel to the wave rows of thecorrugated medium 2 is embossed at the end of one of two trapezoidalshapes D, and a overlap width 7 to the anechoic chamber wall face isformed on the outside thereof.

The sheet materials for assembly 1A, 1B cut as mentioned above areassembled as a wedge form radio wave absorber 20 as shown in FIG. 6, byrespectively folding the sheet material for assembly 1A into the wedgeform along the fold 4 and the sheet material for assembly 1B into thesupport part, and mounting them.

In short, the sheet material for assembly 1A is folded into the wedgeform along the fold 4 of the center and, at the same time, the outershape reinforcing parts 11 on both sides are folded inside at the pointof the fold 4, while the overlap width 7 is folded outside. On the otherside, the trapezoidal shapes D on both sides of the sheet material forassembly 1B are forded into the gate form and, at the same time,respective outer shape reinforcing parts 11 are folded inside, and four(4) insert flap 5 are made to protrude respectively. The support part ofthe sheet material for assembly 1B formed into the gate shape in thismanner is introduced into the leg of the sheet material for assembly 1Afolded into the wedge form and, at the same time, the insert flap 5 atthe end of the one is introduced into the insert slit 6 of the other endof the other, allowing to assemble them as wedge form radio waveabsorber 20 of hollow three-dimensional structure body shown in FIG. 6.The form of the hollow three-dimensional structure body is fixed byintroducing the insert flap 5 into the insert slit 6 in this way,allowing to enhance the field workability dramatically. It goes withoutsaying that they may be assembled with an adhesive agent so long as itdoes not deteriorate the workability.

FIG. 7 shows a wedge form radio wave absorber according to still anotherembodiment of the present invention.

In this radio wave absorber 30 of this embodiment, the sheet forassembly 1 is cut into two, folded into the wedge form respectively,adhered and fixed respectively in parallel on a sintered ferrite plate 8where an aluminum plate 9 is pasted to the back.

Such combination with the sintered ferrite plate permits to cover theradio wave absorption of a specific low frequency range supposed to benecessary especially for an EMC anechoic chamber, thus obtain excellentproperties even when the height of the radio wave absorber is reducedcompared to the wavelength to be absorbed and use as much space of theanechoic chamber as reduced. For instance, there is provided an effectthat a height H of 2 m necessary for the case where the radio waveabsorber is used alone without combining the sintered ferrite plate canbe reduced to 1 m.

FIG. 8 shows a pyramidal form radio wave absorber according to stillanother embodiment of the present invention.

The sheet for assembly 1 for assembling this pyramidal form radio waveabsorber 40 is cut so that four (4) isosceles triangles are connectedalong the folds 4. Angles of the wave rows of the corrugated medium 2 tothe folds of four (4) isosceles triangles are different each other, asthey are cut out from a single sheet material.

FIG. 9 shows a wedge form radio wave absorber according to still anotherembodiment of the present invention.

The radio wave absorber 50 of this embodiment is composed by erectingeight (8) hollow three-dimensional structure bodies made of the sheetfor assembly 1 on a common pedestal 12. The pedestal 12 is composed byarranging an aluminum plate 9 at the lowest portion and laminating three(3) sheet materials for assembly 1 thereon so that wave rows of thecorrugated medium 2 cross each other with right angles between layers.The hollow three-dimensional structure body is composed by folding eight(8) cut sheet materials for assembly 1 respectively in two wedges,making them a set and erecting so that corrugated ridge line directionsof wedges cross each other among adjacent hollow three-dimensionalstructure bodies.

Such arrangement is appropriate for absorbing microwaves to milliwavesof particularly short wavelength, because the pedestal 12 absorbs alittle radio wave that could not be absorbed completely by the hollowthree-dimensional structure body, improving the absorbing propertiesfurthermore. The absorbing properties vary according to whether theelectric field vibration face of incident radio wave and the wedge edgeline are parallel or vertical.

In short, though the wedge form radio wave absorber is essentiallypolarization dependent because of the shape thereof however, thepolarization dependence can be avoided by arranging adjacent wedge formradio wave absorbers so that the wedge edge lines cross each other, asthe embodiment shown in FIG. 9.

It is possible to make the sheet material for a radio wave mabsorber ofthe present invention as a radio wave absorber for milliwave band bylaminating one layer or two or more layers thereon on a reflection flatplate.

A layer height 1 to 5 mm of the corrugated medium of the sheet for radiowave absorber is at the same order as the waveform approximately in themilliwave band, because, for instance, the wave lengths at 30 GHz and100 GHz are respectively 10 mm and 3 mm. The corrugated medium itself ofthe sheet material for a radio wave absorber can function as a wedgeform radio wave absorber for the milliwave, because the wedge form radiowave absorber generally presents better wave absorbing properties fromthe vicinity where the height thereof becomes the same order as thewavelength. It is preferable to compose the corrugated medium with asheet comprising an electrical-loss material in the case of using thesheet material for a radio wave absorber as a sheet for a radio waveabsorber in this manner.

Besides, in the case of composing the radio wave absorber in a sheetstate, it can be so composed to laminate two or more layers and, in thiscase, they may be laminated so that the wave rows of the corrugatedmedium cross each other between layers. Such lamination form can cancelthe polarization dependence of the radio wave absorber. Moreover, in thecase of composing the radio wave absorber in a state of sheet, it may bealso possible to improve properties furthermore, by changing theconcentration of electrical-loss material contained in the corrugatedmedium across the lamination direction.

Embodiment of Inner Radio Wave Absorber (1)

FIG. 10 and FIG. 11 show an embodiment according to the presentinvention. 13 denotes a hollow three-dimensional structure body ofconductive thin member formed into a pyramidal shape or a wedge shape byfolding etc. 14 denotes an inner radio wave absorber made from theconductive thin member.

FIG. 12 is a cross sectional view cut along A-A′-B-B′ surface of theembodiments in FIG. 10 and FIG. 11.

When the inner radio wave absorber 14 is inserted at a position tooclose to the top A-A′, there is only a slight effect since the area ofthe inner radio wave absorber 14 is small. On the other hand, when theinner radio wave absorber 14 is inserted at a position too close to thebottom surface B-B′, an effect of inserting the inner radio waveabsorber 14 is also small due to a boundary condition where electriccomponent of the incident electromagnetic wave becomes smaller whenmounting for B-B′ to contact with the metal plate. In order to increasethe effect of the inner radio wave absorber 14, according to theexperimental result, it is preferable to divide a length from A-A′ toB-B′ into three, then to mount at the middle of the divided positions.

FIG. 13 is also a cross sectional view similar to FIG. 12 and shows anembodiment in which two conductive thin members 14 a and 14 b aremounted as inner radio wave absorbers. Like this example it is possibleto mount a plurality of conductive thin members. As the number of theconductive thin members is larger, it becomes closer condition whereinside of the hollow three-dimensional structure body is filled up withloss material, thus causing a tendency of increasing the radiowave-absorbing property at the high frequency.

Embodiment of Inner Radio Wave Absorber (2)

FIG. 14 shows an embodiment of the present invention. In the structureof the embodiment, two triangular-shaped sheets for a radio waveabsorber 1 a, 1 b two sides of which are the inner wall surface of ahollow three-dimensional structure body 13 of pyramidal shape to matchwith each other at a right angle to form an inner radio wave absorber,and arrange this perpendicularly to the bottom of the pyramid. Thereason to compose two triangular shaped conductive thin members 1 a, 1 bperpendicularly each other is to eliminate a polarization property of anabsorption property against the incident electromagnetic wave.

FIG. 15 shows an example of a structure of the triangular shaped sheetfor a radio wave absorber 1 a, 1 b used in FIG. 14.

1 a provided with a notch part 15 from a peak of the triangle to a basethereof. On the other hand, 1 b provided with a notch part 15 from abase of the triangle to a peak thereof. Each triangle crosses to anothertriangle at the notch part to match the end points P, Q of the notchparts so as to make a right angle between 1 a and 1 b to form an innerradio wave absorber.

Embodiment of Inner Radio Wave Absorber (3)

FIG. 16 shows another embodiment of the present invention. It is astructure in which a piece of conductive thin member of an isoscelestriangle shape is arranged perpendicularly to the ridge line of thewedge inside of the hollow three-dimensional structure body 13 of awedge shape as an inner radio wave absorber 14. The inner radio waveabsorber 14 may be located in any position of ridge line direction ofthe wedge.

FIG. 17 shows a structure in which two inner radio wave absorbers 14 a,14 b in the embodiment illustrated in FIG. 16. As such, it is possibleto mount a plurality of conductive thin members. As the number of theconductive thin members is larger, it becomes closer condition whereinside of the hollow three-dimensional structure body is filled up withloss material, thus causing a tendency of increasing the radiowave-absorbing property at the high frequency.

EMBODIMENTS Embodiment 1

Carbon fiber having sizing agent content rate of 0% and average fiberlength of 12 mm, chopped glass fiber and aramid pulp are mixed in aratio of 3 wt %, 77 wt % and 20 wt % respectively and submitted to thewet paper-making method to obtain an incombustible sheet A including anelectrical-loss material of 0.25 mm in thick, 150 g/m² in basis weight.On the other hand, chopped glass fiber and aramid pulp are mixed in aratio of 80 wt % and 20 wt % respectively and submitted to the wetpaper-making method to obtain an incombustible sheet A not including anelectrical-loss material of 0.25 mm in thick, 150 g/m² in basis weight,and thereafter, a sheet B where a moss green color is printed on oneside of the sheet is obtained. Here, the non-combustibility designatesto be flameproof 1 grade of JIS A 1322 test, combustibility test, or V-0class in UL-94 thin material vertical combustion test.

Thus obtained sheet A is used for the liner on one side and thecorrugated medium, the sheet B is used for the liner of the other sidemaking the printed face outside and the corrugated medium is corrugatedto manufacture a double faced paperboard of 2.5 mm in height, 1.5 timesin take up ratio and 5 mm in interval between adjacent crests. A starchbase adhesive of approximately 5 g/m² is used for affixing thecorrugated medium and the liner. The fire retardancy of the double facedpaperboard was flameproof 1 grade of JIS A 1322 test and V-0 class inUL-94 thin material vertical combustion test.

Next, this paperboard is cut into four (4) sheet materials of 60 cm×201cm, short sides of two of them are joined respectively with a papertape, to make two (2) wedge form assemblies of 30 cm×60 cm bottom side×2m height. These two wedge form assemblies are fixed in parallel on analuminum plate of 60 cm in length×60 cm in width×1 mm in thickness, toobtain a wedge form radio wave absorber of 60 cm×60 cm bottom side×about2 m height.

In the manufacturing process of the aforementioned radio wave absorber,no distortion or the like has occurred in the assembly, and it could beassemble as a radio wave absorber presenting an excellent form-retainingproperty.

In addition, the radio wave-absorbing property of thus obtained radiowave absorber was measured, to obtain an excellent radio wave-absorbingability of −15 dB to −20 dB in a frequency range of 30 MHz to 300 MHz.

It should be appreciated that the radio wave-absorbing ability of thistime was determined from the difference of reflection level by measuringthe reflection of the time when radio wave is applied vertically to analuminum plate of 60 cm in length×60 cm in width×1 mm in thickness, andthat obtained when radio wave is applied similarly to the radio waveabsorber of the same area.

Embodiment 2

A paperboard same as the one manufactured in the embodiment 1 is cutinto four (4) sheet material of 60 cm×101 cm, short sides of two of themare joined respectively with a paper tape, to make two (2) wedge formassemblies of 30 cm×60 cm bottom side×1 m height. These two wedge formassemblies are fixed in a way to arrange on a sintered ferrite plate of60 cm in length×60 cm in width×5.7 mm in thickness, and an aluminumplate of 60 cm in length×60 cm in width×1 mm in thickness is affixed tothe back of the sintered ferrite plate, to obtain a wedge form radiowave absorber of 60 cm×60 cm bottom side×about 1 m height.

In the manufacturing process of the aforementioned radio wave absorber,no distortion or the like has occurred in the assembly, and it could beassembled as a radio wave absorber presenting an excellentform-retaining property. In addition, the radio wave-absorbing propertyof thus obtained radio wave absorber was measured, to obtain anexcellent radio wave-absorbing ability of −15 dB to −20 dB in afrequency range of 30 MHz to 300 MHz.

Embodiment 3

A paperboard same as the one manufactured in the embodiment 1 is cut tohave embossed folds, insert flaps and insert slits as shown in FIG. 3 towork out two (2) sheet materials for assembly. Two (2) wedge formassemblies of 1 m height, 30 cm×60 cm bottom side respectively as shownin FIG. 4 were made from these two (2) sheet materials for assembly.These two wedge form assemblies are fixed in parallel on a sinteredferrite plate same as the embodiment 2, and an aluminum plate same asthe embodiment 2 is affixed to the back of the sintered ferrite plate,to obtain a wedge form radio wave absorber of 60 cm×60 cm bottomside×about 1 m height.

In the manufacturing process of the aforementioned radio wave absorber,no distortion or the like has occurred in the assembly, and it could beassembled as a radio wave absorber presenting an excellentform-retaining property. In addition, the radio wave-absorbing propertyof thus obtained radio wave absorber was measured, to obtain anexcellent radio wave-absorbing ability of −15 dB to −20 dB in afrequency range of 30 MHz to 300 MHz.

Embodiment 4

Carbon fiber having sizing agent content rate of 0% and average fiberlength of 6 mm, chopped glass fiber and aramid pulp are mixed in a ratioof 0.8 wt %, 70 wt % and 29.2 wt % respectively and submitted to the wetpaper-making method to obtain an incombustible sheet C includingelectrical-loss material of 0.25 mm in thick, 150 g/m² in basis weight.On the other hand, chopped glass fiber and aramid pulp are mixed in aratio of 80 wt % and 20 wt % respectively and submitted to the wetpaper-making method to obtain an incombustible sheet A not includingelectrical-loss material of 0.25 mm thick, 150 g/m² basis weight and,thereafter, a sheet D where a moss green color is printed on one side ofthe sheet was obtained.

Thus obtained sheet C is used for the corrugated medium, the sheet D isused for the liners of both sides making the printed face outside andthe corrugated medium is corrugated to manufacture a double facedpaperboard of 2.5 mm in layer height, 1.5 times in take up ratio and 5mm in interval between adjacent crests. A starch base adhesive ofapproximately 5 g/m² is used for affixing the corrugated medium and theliner. The fire retardancy of this double faced paperboard wasflameproof 1 grade of JIS A 1322 test and V-0 class in UL-94 thinmaterial vertical combustion test.

Next, this paperboard is cut into four (4) sheet materials of 60 cm×201cm, short sides of two of them are joined respectively with a papertape, to make two (2) wedge form assemblies of 30 cm×60 cm bottom side×2m height. These two wedge form assemblies are fixed in parallel on analuminum plate of 60 cm in length×60 cm in width×1 mm in thickness, toobtain a wedge form radio wave absorber of 60 cm×60 cm bottom side×about2 m height.

In the manufacturing process of the aforementioned radio wave absorber,no distortion or the like has occurred in the assembly, and it could beassemble as a radio wave absorber presenting an excellent form-retainingproperty. In addition, the radio wave-absorbing property of thusobtained radio wave absorber was measured, to obtain an excellent radiowave-absorbing ability of −20 dB to −30 dB in a frequency range of 30MHz to 300 MHz.

It should be appreciated that the radio wave-absorbing ability of thistime was determined from the difference between a reflection level bymeasuring the reflection of the time when radio wave is appliedvertically to an aluminum plate of 60 cm in length×60 cm in width×1 mmin thickness, and that obtained when radio wave is applied similarly tothe radio wave absorber of the same area.

Embodiment 5

A paperboard same as the one manufactured in the embodiment 4 is cutinto four (4) sheet materials of 60 cm×101 cm, short sides of two ofthem are joined respectively with a paper tape, to make two (2) wedgeform assemblies of 30 cm×60 cm bottom side×1 m height. These two wedgeform assemblies are fixed in a way to arrange on a sintered ferriteplate of 60 cm in length×60 cm in width×5.7 mm in thickness, and analuminum plate of 60 cm in length×60 cm in width×1 mm in thickness isaffixed to the back of the sintered ferrite plate, to obtain a wedgeform radio wave absorber of 60 cm×60 cm bottom side×about 1 m height.

In the manufacturing process of the aforementioned radio wave absorber,no distortion or the like has occurred in the assembly, and it could beassembled as a radio wave absorber presenting an excellentform-retaining property.

The absorbing ability of this radio wave absorber was measured, toobtain −20 dB to −30 dB in 30 MHz to 300 MHz, −20 dB to −25 dB in 300MHz to 1 GHz, and −20 dB to −30 dB in 1 GHz to 18 GHz, confirming thepossibility to obtain an excellent absorbing ability not only in thefrequency band of the anechoic chamber for EMC, 30 MHz to 1 GHz, butalso in the microwave band.

Embodiment 6

Carbon fiber having sizing agent content rate of 0% and average fiberlength of 6 mm chopped glass fiber, aramid pulp and aluminum hydroxideare mixed in a ratio of 0.8 wt %, 50 wt %, 9.2 wt % and 40 wt %respectively and submitted to the wet paper-making method to obtain anincombustible sheet E including electrical-loss material of 0.15 mmthick, 100 g/m² basis weight. On the other hand, chopped glass fiber,aramid pulp and aluminum hydroxide are mixed in a ratio of 50 wt %, 10wt % and 40 wt % respectively and submitted to the wet paper-makingmethod to obtain a sheet F not including electrical-loss material of0.15 mm thick, 100 g/m².

Thus obtained sheet E is used for the corrugated medium, the sheet F isused for the liner and the corrugated medium is corrugated tomanufacture a double faced paperboard of 1.2 mm in height, 1.3 times intake up ratio and 3 mm in interval between adjacent crests. The adhesiveas the embodiment 1 is used for affixing the corrugated medium and theliner. The fire retardancy of the paperboard was flameproof 1 grade andV-0 class similarly to the embodiment 1.

The aforementioned paperboard is cut to have embossed folds, insertflaps and insert slits as shown in FIG. 5 (A), (B) to work out two (2)sheet materials for assembly. Four (4) wedge form assemblies of 45 cmheight, 30 cm×30 cm bottom side respectively as shown in FIG. 6 weremade from these two (2) sheet materials for assembly. These four (4)wedge form assemblies are fixed to a sintered ferrite plate same as theembodiment 2 so that corrugated ridge line directions of wedges ofadjacent edges cross each other, and an aluminum plate same as theembodiment 2 is affixed to the back thereof, to obtain a wedge formradio wave absorber of 60 cm×60 cm bottom side×about 45 cm height.

In the manufacturing process of the aforementioned radio wave absorber,no distortion or the like has occurred in the assembly, and it could beassembled as a radio wave absorber presenting an excellentform-retaining property. In addition, the radio wave-absorbing propertyof thus obtained radio wave absorber was measured, to obtain anexcellent radio wave-absorbing ability of −20 dB to −30 dB in afrequency range of 30 MHz to 300 MHz.

Embodiment 7

The sheet F not including an electrical-loss material manufactured inthe embodiment 6 is used for the liner of one side and the corrugatedmedium, while the sheet E including an electrical-loss material is usedfor the liner of the other side, to manufacture a double facedpaperboard, similarly to the embodiment 6.

The aforementioned paperboard is cut as shown in FIG. 5 (A), (B) to makefour (4) wedge form assemblies of 45 cm height, 30 cm×30 cm bottom siderespectively as shown in FIG. 6. These four (4) wedge form assembliesare fixed to a sintered ferrite plate same as the embodiment 4,similarly to the embodiment 6 and an aluminum plate is affixed, toobtain a wedge form radio wave absorber of 60 cm×60 cm bottom side×45 cmheight.

In the manufacturing process of the aforementioned radio wave absorber,no distortion or the like has occurred in the assembly, and it could beassembled as a radio wave absorber presenting an excellentform-retaining property. In addition, the radio wave-absorbing propertyof thus obtained radio wave absorber was measured, to obtain anexcellent radio wave-absorbing ability of −10 dB to −20 dB in afrequency range of 30 MHz to 300 MHz.

Embodiment 8

Carbon fiber having sizing agent of 0% and average fiber length of 6 mm,chopped glass fiber, aramid pulp and aluminum hydroxide are mixed in aratio of 0.2 wt %, 50 wt %, 9.8 wt % and 40 wt % respectively andsubmitted to the wet paper-making method to obtain a sheet G includingan electrical-loss material of 0.15 mm in thick, 100 g/m² in basisweight.

The aforementioned sheet G and sheet E are used for the inner linerrespectively by one layer and the sheet F is used for the other linersand the corrugated medium to manufacture a triple wall of 2.5 mm inlayer height, 1.5 times in take up ratio of the corrugated medium and 5mm in interval between adjacent crests of the corrugated medium. Thefire retardancy of the triple wall was flameproof 1 grade and V-0 classsimilarly to the embodiment 1. This triple wall is cut by 30 cm squareand fitted to a reflector, while the sheet E layer side directs to thereflector side, to form as pedestal.

On the other hand, the sheet F is used for the corrugated medium and theliner of one side, while the sheet G is used for the liner of the otherside, the corrugated medium is corrugated to manufacture a double facedpaperboard of 1.2 mm in layer height, 1.3 times in take up ratio and 3mm in interval between adjacent crests. The fire retardancy of thisdouble faced paperboard was flameproof 1 grade and V-0 class, similarlyto the embodiment 1. This double faced paperboard is cut into 8 piecesof 15 cm×11 cm, folded and arranged on the aforementioned pedestal,while the sheet G side directs to the reflector side as shown in FIG. 8,to form as radio wave absorber body. The radio wave-absorbing propertyof this radio wave absorber was measured in a microwave band of 2 to 18GHz, to obtain an excellent radio wave-absorbing ability of −20 dB to−30 dB.

Embodiment 9

The assembly sheet for a radio wave absorber of 2.5 mm in layer heightmanufacture in the embodiment 4 is assembled as radio wave absorber bymaking the printed surface as radio wave incident face and fitting theother face to a reflector. The radio wave-absorbing property of thisradio wave absorber was measured to obtain −15 dB to −20 dB in amilliwave band of 75 GHz to 110 GHz. It was confirmed that the assemblysheet for a radio wave absorber of the present invention functions asradio wave absorber in the milliwave band.

Embodiment 10

A hollow three-dimensional structure body of a wedge shape is madeexperimentally and the inner radio wave absorber of the aforementionedfirst embodiment is provided therein. A conductive thin member isobtained by impregnating with carbon powder of 0.6 g/m² in anincombustible paper of 1 mm in thickness. This is folded to shape into awedge shape as shown in FIG. 18 to use as a hollow three-dimensionalstructure 13. In FIG. 18, the height (H), depth (D), and width (W) ofthe structure are 45 cm, 30 cm, and 30 cm, respectively. A conductivethin member of the same composition is molded into a rectangular shapeof 15 cm in width and 30 cm in length to obtain an inner radio waveabsorber 14.

The inner radio wave absorber 14 is mounted at a location where it is ata half of the height (H) from the bottom surface of the wedge.Incidentally, in this embodiment, for reinforcing the mechanicalstrength of the conductive thin member a reinforcing member 16 isadhered on the inner radio wave absorber. The reinforcing member 16 ismade from a paperboard of the height of 2.5 mm which is used forpackaging, and has almost no electroconductivity and is transparent asfor radio waves. Therefore it does not affect any effect on radio wavesgiven by the inner radio wave absorber 14.

Although it is not used in this embodiment, adhering a reinforcingmember 16 to the hollow three-dimensional structure body increases' themechanical strength of the wedge shape. A mechanism for mounting theinner radio wave absorber 14 in the hollow three-dimensional structurebody 13 may be arbitrarily adopted within a range that does not affectthe radio wave-absorbing property. In this embodiment, adhesive is usedfor binding.

Four absorbing bodies of wedge shape shown in FIG. 18 are experimentallymade to alternately arrange the ridge lines of their wedges to form oneunit of radio wave absorber. It is known that a radio wave absorber ofwedge shape has different radio wave absorption amount depending onwhether the electric field vibration direction of incident radio wave isparallel or vertical to the ridge line of wedge. In an arrangement asshown in FIG. 19, average absorption amount of radio wave-absorptionamounts of both directions is obtained and the effect of thedirectionality of the radio wave-absorbing property is eliminated.

The one unit of radio wave absorber shown in FIG. 19 is disposed on aferrite tile absorber 8 to measure its radio wave-absorbing property inthe frequency band of 30 MHz or more. It is said that the ferrite tileabsorber 8 has a good radio wave-absorbing property in a range between30 MHz and several hundreds MHz, however, the radio wave-absorbingproperty can be more improved mainly at 100 MHz or higher by arrangingthe one unit.

FIG. 20 shows a radio wave-absorbing property. In FIG. 20, (1), (2) and(3) respectively indicate a radio wave-absorbing property when using theunit of the aforementioned embodiment on the ferrite plate; a radiowave-absorbing property when using only a hollow three-dimensionalstructure 13 of the present invention; and a radio wave-absorbingproperty of an absorber formed into a wedge shape with the same size asthat of the loss material of this embodiment using loss materialobtained by impregnating a urethane foam material with carbon powder,that is, a wedge shaped radio wave absorber of which the whole volume isoccupied with loss material.

Amount of impregnating carbon in (3) is 6 g/l (carbon powder of 6 g isimpregnated in a 1 liter volume.)

In FIG. 20, (1), (2), and (3) show almost same radio wave-absorbingproperty in the range between 30 MHz and 300 MHz. This results from thefact that the radio wave-absorbing property in this frequency band isdominantly determined by the ferrite tile absorber 8.

However, when radio wave-absorption amount of several hundreds MHz orhigher is noticed, the radio wave-absorption amount is larger in theorder of (3)>(1)>(2). Since (1) and (2) are different on the pointwhether it has the inner radio wave absorber 14, that shows advantage ofarranging an inner radio wave absorber.

INDUSTRIAL APPLICABILITY

A sheet material for a radio wave absorber of the present invention anda radio wave absorber thereof can be used for the walls, ceiling, floorand the like of an anechoic chamber, which is used for measurementtesting the properties of antennae or radio-wave measurement testing anelectronic device, in order to prevent any radio waves from beingentered from outside and from radiating them externally.

In addition to the applications other than anechoic chamber relatedfacilities, it can be used to create environment necessary to shield orabsorb radio waves.

DESCRIPTION OF SYMBOLS

-   1 Sheet material for a radio waveabsorber-   2 Corrugated medium-   3 Liner-   4 Fold-   5 Insert flap-   6 Insert slit-   7 Overlap width-   8 Ferrite plate-   9 Aluminum plate-   10,20,30,40,50 Radio wave absorber-   11 Outer shape reinforcing part-   12 Pedestal-   13 Hollow three-dimensional structure body-   14 Inner radio wave absorber-   15 Notch part-   16 Reinforcing member

1. A sheet material for a radio wave absorber comprising a paperboardstructure in which a corrugated medium and a planar liner are layeredover each other, wherein the corrugated medium and/or the liner areconstructed from a sheet including an electrical-loss material.
 2. Thesheet material for a radio wave absorber of claim 1, wherein theelectrical-loss material is an electroconductive fiber.
 3. The sheetmaterial for a radio wave absorber of claim 2, wherein the sheet ismixed paper including the electroconductive fiber.
 4. The sheet materialfor a radio wave absorber of claim 3, wherein a ratio (y/p) of maximumelectric conductivity (p) of the mixed paper and electric conductivity(y) measured in a direction orthogonal to a measurement directionpresenting the maximum electric conductivity (p) is in a range of 0.35to 0.95.
 5. The sheet material for a radio wave absorber of claim 1,wherein the paperboard structure is one selected from single facedpaperboard, double faced paperboard, double wall paperboard and triplewall.
 6. The sheet material for a radio wave absorber of claim 1,wherein thickness per a layer of the paperboard structure is 1 to 5 mm.7. The sheet material for a radio wave absorber of claim 1, wherein thetake up ratio of the corrugated medium to the liner of the paperboardstructure is in a range of 1.2 to 2 times, and the interval between topsof adjacent corrugated mediums is in a range of 1 to 15 mm.
 8. The sheetmaterial for a radio wave absorber of claim 3, wherein theelectroconductive fiber is a carbon fiber, an average fiber length ofthe carbon fiber is 1 to 60 mm and a mixing ratio in the mixed paper is0.08 to 20 wt %.
 9. The sheet material for a radio wave absorber ofclaim 8, wherein a content of sizing agent adhered to the carbon fiberis not more than 0.9 wt % of total carbon fiber weight.
 10. The sheetmaterial for a radio wave absorber of claim 1, wherein at least oneselected from the group consisting of printing of colors, patterns orletters, and embossing of patterns or letters is applied to an outsidesurface of the liner.
 11. A radio wave absorber, wherein the sheetmaterial for a radio wave absorber of claim 1 is cut, folded, andassembled into a hollow three-dimensional structure body, which has ashape of wedge, polygonal-pyramid, or polygonal cylinder.
 12. A radiowave absorber, wherein, inside of the hollow three-dimensional structurebody of claim 11, one or more of a sheet material for a radio waveabsorber comprising a paperboard structure in which a corrugated mediumand a planar liner are layered over each other, wherein the corrugatedmedium and/or the liner are constructed from a sheet including anelectrical-loss material, is arranged parallel to a bottom surface ofthe radio wave absorber.
 13. A radio wave absorber of which the hollowthree-dimensional structure body of claim 11 has a pyramidal form,wherein a sheet material for a radio wave absorber comprising apaperboard structure in which a corrugated medium and a planar liner arelayered over each other, wherein the corrugated medium and/or the linerare constructed from a sheet including an electrical-loss material, isformed into an isosceles triangle plate two sides of which are along aninner wall of the radio wave absorber to match with each other at aright angle, and the other side of which is arranged perpendicularly tothe bottom surface of the radio wave absorber.
 14. A radio wave absorberof which the hollow three-dimensional structure body of claim 11 has awedge form, wherein, inside of the radio wave absorber, a sheet materialfor a radio wave absorber comprising a paperboard structure in which acorrugated medium and a planar liner are layered over each other,wherein the corrugated medium and/or the liner are constructed from asheet including an electrical-loss material, is formed into an isoscelestriangle plate two sides of which are along an inner wall of the radiowave absorber to arrange one or more plates perpendicularly to a ridgeline of wedge.
 15. The radio wave absorber of claim 11, wherein a sheetmaterial for a radio wave absorber comprising a paperboard structure inwhich a corrugated medium and a planar liner are layered over eachother, wherein the corrugated medium and/or the liner are constructedfrom a sheet including an electrical-loss material, has paired insertslits and insert flaps, and the hollow three-dimensional structure bodyis assembled by inserting the insert flap into the insert slits not todeform the shape.
 16. The radio wave absorber of claim 11, wherein thehollow three-dimensional structure body is erected on a sintered ferriteplate.
 17. The radio wave absorber of claim 11, wherein the hollowthree-dimensional structure body is erected on a pedestal where a sheetmaterial for a radio wave absorber comprising a paperboard structure inwhich a corrugated medium and a planar liner are layered over eachother, wherein the corrugated medium and/or the liner are constructedfrom a sheet including an electrical-loss material, is layered over inone or more layers.
 18. The radio wave absorber of claim 11, wherein thepedestal is formed by layering on a reflective flat plate one or moresheet material layers for a sheet material for a radio wave absorbercomprising a paperboard structure in which a corrugated medium and aplanar liner are layered over each other, wherein the corrugated mediumand/or the liner are constructed from a sheet including anelectrical-loss material, where at least the corrugated medium is formedfrom a sheet including the electrical-loss material, and the hollowthree-dimensional structure body is erected on the pedestal.
 19. Theradio wave absorber of claim 17, wherein two or more layers of the sheetmaterial for a radio wave absorber are layered over so that a corrugatedrow direction of the corrugated medium crosses each other among thelayers.